1. Field of the Invention
The present invention is generally related to circuits that require a capacitor to be discharged over a precisely determined period of time and, more particularly, to a circuit in which a very small, but accurately determined electrical current is provided to discharge a capacitor in a peak detecting circuit.
2. Description of the Prior Art
Many different types of peak detecting circuits are known to those skilled in the art. Some circuits of this type are used in association with gear tooth sensors that measure relative change in magnetic field strength as a result of the passage of ferromagnetic discontinuities through a detection zone proximate a magnetically sensitive device, such as a Hall effect element. The basic theory of operation of a device of this type is to provide a magnetically sensitive component with a bias magnet which causes the magnetically sensitive component to provide an output signal that is responsive to the strength of a magnetic field perpendicular to its plane. Although the magnetically sensitive device can be a magnetoresistive circuit, it is common to use Hall effect elements for these purposes. A rotating device, such as a gear, has a plurality of ferromagnetic discontinuities, or teeth. As the teeth move through the detection zone, the magnetic field is disturbed and caused to change from one state to another as the teeth and interstitial slots move through the detection zone. Often, devices of this type operate under the principle that a tooth will cause the output signal from the Hall device to change in one direction relative to an average signal from both a tooth and a slot while the interstitial slot will cause the output signal from the Hall device to move in the opposite direction. In this way, a circuit associated with the gear tooth sensor determines an average signal level for the teeth and the slots and then compares the instantaneous signal to the average in order to determine whether a tooth or a slot is moving through the detection zone. Alternative circuit configurations store minimum values and create reference thresholds based on the minimum values. In other words, if the minimum value of 1500 gauss is stored by a monitoring circuit, it would be possible to use a value of 1560 gauss as a threshold to trigger a digital output signal. This threshold would be selected by adding 60 gauss to the minimum stored value. Circuits of these types can use either average values, minimum values or maximum values as the stored signal level that is used to provide a reference signal or threshold magnitude. These circuits are well known to those skilled in the art.
U.S. Pat. No. 4,992,731, which issued to Lorenzen on Feb. 12, 1991, describes a rotary speed sensor with base line compensation of a Hall cell output signal. The rotary speed sensor described in the Lorenzen patent uses a permanent magnet and a Hall cell which is sensitive to the changing tangential component of magnetic field caused by the interrupted surface profile of a rotary element. In order to avoid anomalies caused by the variation of the base value of the tangential component of magnetic field, the output of the differential amplifier is feed by the Hall cell and is connected to a voltage averaging circuit which stores the average voltage of the output on a single capacitor. The output of the differential amplifier and the voltage across the capacitor are respectively coupled to the differential inputs of a Schmitt trigger exhibiting hysteresis. The Schmitt trigger thereby establishes operate and release points relative to the average voltage signal. The voltage averaging circuit is a nonlinear circuit in which the voltage across the capacitor is feed back to the negative input of an operational amplifier. A power-up circuit is also provided for rapidly charging the capacitor initially to approximately the average voltage.
In circuits of the type described immediately above, it is necessary for the average signal to be available as a reference when the next tooth passes through the detection zone. Otherwise, the comparison between the instantaneous signal and the average signal can not appropriately be made. U.S. Pat. No. 4,992,731, which is described above, takes steps to rapidly charge the capacitor initially, during startup, to a magnitude which is approximately equivalent to the average voltage that will be stored by the capacitor during normal operation. However, several other problems can occur in the operation of a circuit of this type or the type that stores a maximum or minimum value that is used as a bases for determining a threshold magnitude. For example, electrical noise can cause the voltage signal stored by the capacitor to be excessively small or excessively large during either start-up or during later operation.
In order to assure that a proper value is available for use as a reference as each subsequent ferromagnetic discontinuity, or tooth, moves toward the detection zone, it would be advantageous if a means was provided to discharge the capacitor over a preselected and precise period of time and at the same time provide a buffer or detector for the capacitor voltage.